The spinal column of vertebrates provides support to bear weight and protection to the delicate spinal cord and spinal nerves. The spinal column includes a series of vertebrae stacked on top of each other. There are typically seven cervical (neck), twelve thoracic (chest), and five lumbar (low back) segments. Each vertebra has a cylindrical shaped vertebral body in the anterior portion of the spine with an arch of bone to the posterior, which covers the neural structures. Between each vertebral body is an intervertebral disc, a cartilaginous cushion to help absorb impact and dampen compressive forces on the spine. To the posterior the laminar arch covers the neural structures of the spinal cord and nerves for protection. At the junction of the arch and posterior vertebral body are articulations to allow movement of the spine.
Various types of problems can affect the structure and function of the spinal column. These can be based on degenerative conditions of the intervertebral disc or the articulating joints, traumatic disruption of the disc, bone or ligaments supporting the spine, tumor or infection. In addition congenital or acquired deformities can cause abnormal angulation or slippage of the spine. Slippage (spondylolisthesis) anterior of one vertebral body on another can cause compression of the spinal cord or nerves. Patients who suffer from one of more of these conditions often experience extreme and debilitating pain, and can sustain permanent neurological damage if the conditions are not treated appropriately.
One technique of treating these disorders is known as surgical arthrodesis of the spine. This can be accomplished by removing the intervertebral disc and replacing it with bone and immobilizing the spine to allow the eventual fusion or growth of the bone material across the disc space to connect the adjoining vertebral bodies together. The stabilization of the vertebra to allow fusion is often assisted by a surgically implanted device to hold the vertebral bodies in proper alignment and allow the bone to heal, much like placing a cast on a fractured bone. Such techniques have been effectively used to treat the above-described conditions and in most cases are effective at reducing the patient's pain and preventing neurological loss of function. However, there are disadvantages to the present stabilization devices.
Several types of anterior spinal fixation devices are in use currently. One technique involves placement of screws all the way through the vertebral body, called bicortical purchase. The screws are placed through a titanium plate but are not attached to the plate. This device is difficult to place, and over penetration of the screws can result in damage to the spinal cord. The screws can back out of the plate into the surrounding tissues, as they do not fix to the plate. Several newer generation devices have used a unicortical purchase of the bone, and in some fashion locking the screw to the plate to provide stability and secure the screw from back out. Problems have resulted from over rigid fixation and stress shielding, resulting in nonunion of the bony fusion, chronic micro-motion during healing resulting in stress fracture of the fixation device at either the screw or the plate, insecure locking of the screw to the plate resulting in screw back out, or inadequate fixation strength and resultant collapse of the graft and angulation of the spine.
These devices are often designed to support and bridge across a group of vertebrae, for example a group of three. Because these devices are typically bridged across the bone, for example in the cervical region, they occasionally aggravate the esophagus, making it difficult for one to swallow food. In addition, the screws are installed into the bone normal, i.e., 90° to the plate's surface. Local angularity in the vertebral column often causes high shearing stresses to be applied to the screws. These stresses may fatigue the screws or cause deformation of the screw holes.